The present invention relates to a thermal ink jet printhead, to a printer system incorporating such a printhead, and to a method of ejecting a liquid drop (such as an ink drop) using such a printhead.
The present invention involves the ejection of ink drops by way of forming gas or vapor bubbles in a bubble forming liquid. This principle is generally described in U.S. Pat. No. 3,747,120 (Stemme).
There are various known types of thermal ink jet (bubblejet) printhead devices. Two typical devices of this type, one made by Hewlett Packard and the other by Canon, have ink ejection nozzles and chambers for storing ink adjacent the nozzles. Each chamber is covered by a so-called nozzle plate, which is a separately fabricated item and which is mechanically secured to the walls of the chamber. In certain prior art devices, the top plate is made of Kapton(trademark) which is a Dupont trade name for a polyimide film, which has been laser-drilled to form the nozzles. These devices also include heater elements in thermal contact with ink that is disposed adjacent the nozzles, for heating the ink thereby forming gas bubbles in the ink. The gas bubbles generate pressures in the ink causing ink drops to be ejected through the nozzles.
It is an object of the present invention to provide a useful alternative to the known printheads, printer systems, or methods of ejecting drops of ink and other related liquids, which have advantages as described herein.
According to a first aspect of the invention there is provided an ink jet printhead comprising:
a plurality of nozzles; and
at least one respective heater element corresponding to each nozzle, wherein
each heater element is substantially covered by a conformal protective coating, the coating of each heater element having been applied substantially to all sides of the heater element simultaneously such that the coating is seamless;
each heater element is arranged for being in thermal contact with a bubble forming liquid, and
each heater element is configured to heat at least part of the bubble forming liquid to a temperature above its boiling point to form a gas bubble therein thereby to cause the ejection of a drop of an ejectable liquid through the nozzle corresponding to that heater element.
According to a second aspect of the invention there is provided a printer system incorporating a printhead, the printhead comprising:
a plurality of nozzles; and
at least one respective heater element corresponding to each nozzle, wherein
each heater element is substantially covered by a conformal protective coating, the coating of each heater element having been applied substantially to all sides of the heater element simultaneously such that the coating is seamless;
each heater element is arranged for being in thermal contact with a bubble forming liquid, and
each heater element is configured to heat at least part of the bubble forming liquid to a temperature above its boiling point to form a gas bubble therein thereby to cause the ejection of a drop of an ejectable liquid through the nozzle corresponding to that heater element.
According to a third aspect of the invention there is provided a method of ejecting a drop of an ejectable liquid from a printhead, the printhead comprising a plurality of nozzles and at least one respective heater element corresponding to each nozzle, the method comprising the steps of:
providing the printhead, including applying to each heater element, substantially to all sides thereof simultaneously, a conformal protective coating such that the coating is seamless;
heating at least one heater element corresponding to a said nozzle so as to heat at least part of a bubble forming liquid which is in thermal contact with the at least one heated heater element to a temperature above the boiling point of the bubble forming liquid;
generating a gas bubble in the bubble forming liquid by said step of heating; and
causing the drop of ejectable liquid to be ejected through the nozzle corresponding to the at least one heated heater element by said step of generating a gas bubble.
As will be understood by those skilled in the art, the ejection of a drop of the ejectable liquid as described herein, is caused by the generation of a vapor bubble in a bubble forming liquid, which, in embodiments, is the same body of liquid as the ejectable liquid. The generated bubble causes an increase in pressure in ejectable liquid, which forces the drop through the relevant nozzle. The bubble is generated by Joule heating of a heater element which is in thermal contact with the ink. The electrical pulse applied to the heater is of brief duration, typically less than 2 microseconds. Due to stored heat in the liquid, the bubble expands for a few microseconds after the heater pulse is turned off. As the vapor cools, it recondenses, resulting in bubble collapse. The bubble collapses to a point determined by the dynamic interplay of inertia and surface tension of the ink. In this specification, such a point is referred to as the xe2x80x9cpoint of collapsexe2x80x9d of the bubble.
The printhead according to the invention comprises a plurality of nozzles, as well as a chamber and one or more heater elements corresponding to each nozzle. Each portion of the printhead pertaining to a single nozzle, its chamber and its one or more elements, is referred to herein as a xe2x80x9cunit cellxe2x80x9d.
In this specification, where reference is made to parts being in thermal contact with each other, this means that they are positioned relative to each other such that, when one of the parts is heated, it is capable of heating the other part, even though the parts, themselves, might not be in physical contact with each other.
Also, the term xe2x80x9cinkxe2x80x9d is used to signify any ejectable liquid, and is not limited to conventional inks containing colored dyes. Examples of non-colored inks include fixatives, infra-red absorber inks, functionalized chemicals, adhesives, biological fluids, water and other solvents, and so on. The ink or ejectable liquid also need not necessarily be a strictly a liquid, and may contain a suspension of solid particles or be solid at room temperature and liquid at the ejection temperature.
In this specification, the term xe2x80x9cperiodic elementxe2x80x9d refers to an element of a type reflected in the periodic table of elements.